N-carboxyanhydride-based-scale synthesis of elamipretide

ABSTRACT

Disclosed are methods of making elamipretide (MTP-131), a peptide compound with therapeutic potential for treating various mitochondrial myopathies. The synthesis of the peptide can be achieved via the use of N-carboxyanhydride-modified amino acid residues, which increases the efficiency of the synthetic process and the purity of the peptide product generated.

RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalPatent Application Ser. No. 62/375,579, filed Aug. 16, 2016.

BACKGROUND OF THE INVENTION

Elamipretide (MTP-131) is a mitochondria-targeting peptide compound withtherapeutic potential for treating diseases associated withmitochondrial dysfunction. Elamipretide contains four-amino acidresidues, and has been synthesized according to typical linear andconvergent solution phase peptide synthesis methods. However, thesynthetic route to generate Elamipretide that has been in use to daterequires the preparation of various differentially protected peptides,such that certain protecting groups may be removed in order to subjectthe deprotected compound to peptide coupling, while other protectinggroups must remain intact. Thus, there exists a need to develop newsynthetic methods to Elamipretide that decrease the need for suchiterative protection and deprotection steps.

SUMMARY OF THE INVENTION

Disclosed are convergent and linear methods of synthesizing Elamipretideusing N-carboxyanhydride (NCA) based reagents to install amino acidresidues. The methods described herein present numerous advantages,particularly in a scaled-up synthesis of Elamipretide, because theNCA-based reagents obviate the need for protecting groups at each stepof peptide synthesis, and the NCA-based reagent provides an activatedform of the corresponding amino acid, which dispenses with the need touse activating peptide coupling reagents.

DETAILED DESCRIPTION OF THE INVENTION

Elamipretide has been shown to have various therapeutic effects indiseases related to mitochondrial dysfunction. Previous synthetic routesto Elamipretide presented challenges with respect to scale-up due toreliance on protected amino acid reagents and several deprotectionsteps.

N-carboxyanhydrides (NCAs) have previously been used in solid phasepeptide synthesis on a small laboratory scale. Typically, however, NCAshave been regarded as poorly suited for the synthesis ofenantiomerically enriched peptides since NCAs are prone to racemization.The present invention is based on the surprising discovery that the useof N-carboxyanhydride analogs of amino acid residues in the synthesis ofElamipretide improves the efficiency of the process and the purityprofile of the final product, without the drawbacks typically associatedwith use of NCAs.

Definitions

The articles “a” and “an” are used herein to refer to one or to morethan one (i.e., to at least one) of the grammatical object of thearticle. By way of example, “an element” means one element or more thanone element.

The term “heteroatom” is art-recognized and refers to an atom of anyelement other than carbon or hydrogen. Illustrative heteroatoms includeboron, nitrogen, oxygen, phosphorus, sulfur and selenium.

The term “alkyl” as used herein is a term of art and refers to saturatedaliphatic groups, including straight-chain alkyl groups, branched-chainalkyl groups, cycloalkyl (alicyclic) groups, alkyl substitutedcycloalkyl groups, and cycloalkyl substituted alkyl groups. In certainembodiments, a straight-chain or branched-chain alkyl has about 30 orfewer carbon atoms in its backbone (e.g., C₁-C₃₀ for straight chain,C₃-C₃₀ for branched chain), and alternatively, about 20 or fewer, or 10or fewer (i.e., C₁-C₁₀). Representative examples of alkyl include, butare not limited to, methyl, ethyl, n-propyl, iso-propyl, n-butyl,sec-butyl, iso-butyl, tert-butyl, n-pentyl, isopentyl, neopentyl, andn-hexyl.

The term “cycloalkyl” means mono- or bicyclic or bridged saturatedcarbocyclic rings, each having from 3 to 12 carbon atoms. Certaincycloalkyls have from 5-12 carbon atoms in their ring structure, and mayhave 6-10 carbons in the ring structure.

The term “heterocycloalkyl” as used herein refers to a radical of anon-aromatic ring system, including, but not limited to, monocyclic,bicyclic, and tricyclic rings, which can be completely saturated orwhich can contain one or more units of unsaturation, for the avoidanceof doubt, the degree of unsaturation does not result in an aromatic ringsystem, and having 3 to 12 atoms including at least one heteroatom, suchas nitrogen, oxygen, or sulfur. For purposes of exemplification, whichshould not be construed as limiting the scope of this invention, thefollowing are examples of heterocyclic rings: aziridinyl, azirinyl,oxiranyl, thiiranyl, thiirenyl, dioxiranyl, diazirinyl, azetyl,oxetanyl, oxetyl, thietanyl, thietyl, diazetidinyl, dioxetanyl,dioxetenyl, dithietanyl, dithietyl, furyl, dioxalanyl, pyrrolyl,oxazolyl, thiazolyl, imidazolyl, oxadiazolyl, thiadiazolyl, triazolyl,triazinyl, isothiazolyl, isoxazolyl, thiophenyl, pyrazolyl, tetrazolyl,pyridyl, pyridazinyl, pyrimidinyl, pyrazinyl, triazinyl, tetrazinyl,quinolinyl, isoquinolinyl, quinoxalinyl, quinazolinyl, pyridopyrazinyl,benzoxazolyl, benzothiophenyl, benzimidazolyl, benzothiazolyl,benzoxadiazolyl, benzthiadiazolyl, indolyl, benztriazolyl,naphthyridinyl, azepines, azetidinyl, morpholinyl, oxopiperidinyl,oxopyrrolidinyl, piperazinyl, piperidinyl, pyrrolidinyl, quinicludinyl,thiomorpholinyl, tetrahydropyranyl and tetrahydrofuranyl.

The term “(cycloalkyl)alkyl” as used herein refers to an alkyl groupsubstituted with one or more cycloalkyl groups.

The term “(heterocycloalkyl)alkyl” as used herein refers to an alkylgroup substituted with one or more heterocycloalkyl (i.e., heterocyclyl)groups.

The term “alkenyl” as used herein means a straight or branched chainhydrocarbon radical containing from 2 to 10 carbons and containing atleast one carbon-carbon double bond formed by the removal of twohydrogens. Representative examples of alkenyl include, but are notlimited to, ethenyl, 2-propenyl, 2-methyl-2-propenyl, 3-butenyl,4-pentenyl, 5-hexenyl, 2-heptenyl, 2-methyl-1-heptenyl, and 3-decenyl.The unsaturated bond(s) of the alkenyl group can be located anywhere inthe moiety and can have either the (Z) or the (E) configuration aboutthe double bond(s).

The term “alkynyl” as used herein means a straight or branched chainhydrocarbon radical containing from 2 to 10 carbon atoms and containingat least one carbon-carbon triple bond. Representative examples ofalkynyl include, but are not limited, to acetylenyl, 1-propynyl,2-propynyl, 3-butynyl, 2-pentynyl, and 1-butynyl.

The term “alkylene” is art-recognized, and as used herein pertains to adiradical obtained by removing two hydrogen atoms of an alkyl group, asdefined above. In one embodiment an alkylene refers to a disubstitutedalkane, i.e., an alkane substituted at two positions with substituentssuch as halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl,hydroxyl, alkoxyl, amino, nitro, sulfhydryl, imino, amido, phosphonate,phosphinate, carbonyl, carboxyl, silyl, ether, alkylthio, sulfonyl,sulfonamido, ketone, aldehyde, ester, heterocyclyl, aromatic orheteroaromatic moieties, fluoroalkyl (such as trifluromethyl), cyano, orthe like. That is, in one embodiment, a “substituted alkyl” is an“alkylene”.

The term “amino” is a term of art and as used herein refers to bothunsubstituted and substituted amines, e.g., a moiety that may berepresented by the general formulas:

wherein R_(a), R_(b), and R_(c) each independently represent a hydrogen,an alkyl, an alkenyl, —(CH₂)_(x)—R_(d), or R_(a) and R_(b), takentogether with the N atom to which they are attached complete aheterocycle having from 4 to 8 atoms in the ring structure; R_(d)represents an aryl, a cycloalkyl, a cycloalkenyl, a heterocyclyl or apolycyclyl; and x is zero or an integer in the range of 1 to 8. Incertain embodiments, only one of R_(a) or R_(b) may be a carbonyl, e.g.,R_(a), R_(b), and the nitrogen together do not form an imide. In otherembodiments, R_(a) and R_(b) (and optionally Re) each independentlyrepresent a hydrogen, an alkyl, an alkenyl, or —(CH₂)_(x)—R_(d). Incertain embodiments, the term “amino” refers to —NH₂.

The term “amido”, as used herein, means —NHC(═O)—, wherein the amidogroup is bound to the parent molecular moiety through the nitrogen.Examples of amido include alkylamido such as CH₃C(═O)N(H)— andCH₃CH₂C(═O)N(H)—.

The term “acyl” is a term of art and as used herein refers to any groupor radical of the form RCO— where R is any organic group, e.g., alkyl,aryl, heteroaryl, aralkyl, and heteroaralkyl. Representative acyl groupsinclude acetyl, benzoyl, and malonyl.

The term “aminoalkyl” as used herein refers to an alkyl groupsubstituted with one or more one amino groups. In one embodiment, theterm “aminoalkyl” refers to an aminomethyl group.

The term “aminoacyl” is a term of art and as used herein refers to anacyl group substituted with one or more amino groups.

The term “aminothionyl” as used herein refers to an analog of anaminoacyl in which the O of RC(O)— has been replaced by sulfur, hence isof the form RC(S)—.

The term “phosphoryl” is a term of art and as used herein may in generalbe represented by the formula:

wherein Q50 represents S or O, and R59 represents hydrogen, a loweralkyl or an aryl; for example, —P(O)(OMe)- or —P(O)(OH)₂. When used tosubstitute, e.g., an alkyl, the phosphoryl group of the phosphorylalkylmay be represented by the general formulas:

wherein Q50 and R59, each independently, are defined above, and Q51represents O, S or N; for example, —O—P(O)(OH)OMe or —NH—P(O)(OH)₂. WhenQ50 is S, the phosphoryl moiety is a “phosphorothioate.”

The term “aminophosphoryl” as used herein refers to a phosphoryl groupsubstituted with at least one amino group, as defined herein; forexample, —P(O)(OH)NMe₂.

The term “azide” or “azido”, as used herein, means an —N₃ group.

The term “carbonyl” as used herein refers to —C(═O)—.

The term “thiocarbonyl” as used herein refers to —C(═S)—.

The term “alkylphosphoryl” as used herein refers to a phosphoryl groupsubstituted with at least one alkyl group, as defined herein; forexample, —P(O)(OH)Me.

The term “alkylthio” as used herein refers to alkyl-S—.

The term “carboxy”, as used herein, means a —CO₂H group.

The term “aryl” is a term of art and as used herein refers to includesmonocyclic, bicyclic and polycyclic aromatic hydrocarbon groups, forexample, benzene, naphthalene, anthracene, and pyrene. The aromatic ringmay be substituted at one or more ring positions with one or moresubstituents, such as halogen, azide, alkyl, aralkyl, alkenyl, alkynyl,cycloalkyl, hydroxyl, alkoxyl, amino, nitro, sulfhydryl, imino, amido,phosphonate, phosphinate, carbonyl, carboxyl, silyl, ether, alkylthio,sulfonyl, sulfonamido, ketone, aldehyde, ester, heterocyclyl, aromaticor heteroaromatic moieties, fluoroalkyl (such as trifluromethyl), cyano,or the like. The term “aryl” also includes polycyclic ring systemshaving two or more cyclic rings in which two or more carbons are commonto two adjoining rings (the rings are “fused rings”) wherein at leastone of the rings is an aromatic hydrocarbon, e.g., the other cyclicrings may be cycloalkyls, cycloalkenyls, cycloalkynyls, aryls,heteroaryls, and/or heterocyclyls. In certain embodiments, aryl refersto a 5-10 membered ring. Alternatively, the term “aryl” refers to aphenyl group.

The term “heteroaryl” is a term of art and as used herein refers to amonocyclic, bicyclic, and polycyclic aromatic group having 3 to 12 totalatoms including one or more heteroatoms such as nitrogen, oxygen, orsulfur in the ring structure, for example, azaindolyl, benzo(b)thienyl,benzimidazolyl, benzofuranyl, benzoxazolyl, benzothiazolyl,benzothiadiazolyl, benzotriazolyl, benzoxadiazolyl, furanyl, imidazolyl,imidazopyridinyl, indolyl, indolinyl, indazolyl, isoindolinyl,isoxazolyl, isothiazolyl, isoquinolinyl, oxadiazolyl, oxazolyl, purinyl,pyranyl, pyrazinyl, pyrazolyl, pyridinyl, pyrimidinyl, pyrrolyl,pyrrolo[2,3-d]pyrimidinyl, pyrazolo[3,4-d]pyrimidinyl, quinolinyl,quinazolinyl, triazolyl, thiazolyl, thiophenyl, tetrahydroindolyl,tetrazolyl, thiadiazolyl, thienyl, thiomorpholinyl, triazolyl ortropanyl, and the like. The “heteroaryl” may be substituted at one ormore ring positions with one or more substituents such as halogen,azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl,amino, nitro, sulfhydryl, imino, amido, phosphonate, phosphinate,carbonyl, carboxyl, silyl, ether, alkylthio, sulfonyl, sulfonamido,ketone, aldehyde, ester, heterocyclyl, aromatic or heteroaromaticmoieties, fluoroalkyl (such as trifluromethyl), cyano, or the like. Theterm “heteroaryl” also includes polycyclic ring systems having two ormore cyclic rings in which two or more carbons are common to twoadjoining rings (the rings are “fused rings”) wherein at least one ofthe rings is an aromatic group having one or more heteroatoms in thering structure, e.g., the other cyclic rings may be cycloalkyls,cycloalkenyls, cycloalkynyls, aryls, heteroaryls, and/or heterocyclyls.

The term “aralkyl” or “arylalkyl” is a term of art and as used hereinrefers to an alkyl group substituted with an aryl group, wherein themoiety is appended to the parent molecule through the alkyl group.

The term “heteroaralkyl” or “heteroarylalkyl” is a term of art and asused herein refers to an alkyl group substituted with a heteroarylgroup, appended to the parent molecular moiety through the alkyl group.

The term “alkoxy” as used herein means an alkyl group, as definedherein, appended to the parent molecular moiety through an oxygen atom.Representative examples of alkoxy include, but are not limited to,methoxy, ethoxy, propoxy, 2-propoxy, butoxy, tert-butoxy, pentyloxy, andhexyloxy.

The term “alkoxycarbonyl” means an alkoxy group, as defined herein,appended to the parent molecular moiety through a carbonyl group,represented by —C(═O)—, as defined herein. Representative examples ofalkoxycarbonyl include, but are not limited to, methoxycarbonyl,ethoxycarbonyl, and tert-butoxycarbonyl.

The term “alkylcarbonyl”, as used herein, means an alkyl group, asdefined herein, appended to the parent molecular moiety through acarbonyl group, as defined herein.

Representative examples of alkylcarbonyl include, but are not limitedto, acetyl, 1-oxopropyl, 2,2-dimethyl-1-oxopropyl, 1-oxobutyl, and1-oxopentyl.

The term “arylcarbonyl”, as used herein, means an aryl group, as definedherein, appended to the parent molecular moiety through a carbonylgroup, as defined herein. Representative examples of arylcarbonylinclude, but are not limited to, benzoyl and (2-pyridinyl)carbonyl.

The term “alkylcarbonyloxy” and “arylcarbonyloxy”, as used herein, meansan alkylcarbonyl or arylcarbonyl group, as defined herein, appended tothe parent molecular moiety through an oxygen atom. Representativeexamples of alkylcarbonyloxy include, but are not limited to, acetyloxy,ethylcarbonyloxy, and tert-butylcarbonyloxy. Representative examples ofarylcarbonyloxy include, but are not limited to phenylcarbonyloxy.

The term “alkenoxy” or “alkenoxyl” means an alkenyl group, as definedherein, appended to the parent molecular moiety through an oxygen atom.Representative examples of alkenoxyl include, but are not limited to,2-propen-1-oxyl (i.e., CH₂═CH—CH₂—O—) and vinyloxy (i.e., CH₂═CH—O—).

The term “aryloxy” as used herein means an aryl group, as definedherein, appended to the parent molecular moiety through an oxygen atom.

The term “heteroaryloxy” as used herein means a heteroaryl group, asdefined herein, appended to the parent molecular moiety through anoxygen atom.

The term “carbocyclyl” as used herein means a monocyclic or multicyclic(e.g., bicyclic, tricyclic, etc.) hydrocarbon radical containing from 3to 12 carbon atoms that is completely saturated or has one or moreunsaturated bonds, and for the avoidance of doubt, the degree ofunsaturation does not result in an aromatic ring system (e.g., phenyl).Examples of carbocyclyl groups include 1-cyclopropyl, 1-cyclobutyl,2-cyclopentyl, 1-cyclopentenyl, 3-cyclohexyl, 1-cyclohexenyl and2-cyclopentenylmethyl.

The term “cyano” is a term of art and as used herein refers to —CN.

The term “halo” is a term of art and as used herein refers to —F, —Cl,—Br, or —I.

The term “haloalkyl” as used herein refers to an alkyl group, as definedherein, wherein some or all of the hydrogens are replaced with halogenatoms.

The term “hydroxy” is a term of art and as used herein refers to —OH.

The term “hydroxyalkyl”, as used herein, means at least one hydroxygroup, as defined herein, is appended to the parent molecular moietythrough an alkyl group, as defined herein. Representative examples ofhydroxyalkyl include, but are not limited to, hydroxymethyl,2-hydroxyethyl, 3-hydroxypropyl, 2,3-dihydroxypentyl, and2-ethyl-4-hydroxyheptyl.

The term “silyl”, as used herein, includes hydrocarbyl derivatives ofthe silyl (H₃Si—) group (i.e., (hydrocarbyl)₃Si—), wherein a hydrocarbylgroups are univalent groups formed by removing a hydrogen atom from ahydrocarbon, e.g., ethyl, phenyl. The hydrocarbyl groups can becombinations of differing groups which can be varied in order to providea number of silyl groups, such as trimethylsilyl (TMS),tert-butyldiphenylsilyl (TBDPS), tert-butyldimethylsilyl (TBS/TBDMS),triisopropylsilyl (TIPS), and [2-(trimethyl silyl)ethoxy]methyl (SEM).

The term “silyloxy”, as used herein, means a silyl group, as definedherein, is appended to the parent molecule through an oxygen atom.

Certain compounds made by methods of the present invention may exist inparticular geometric or stereoisomeric forms. In addition, compounds ofthe present invention may also be optically active. The presentinvention contemplates all such compounds, including cis- andtrans-isomers, (R)- and (S)-enantiomers, diastereoisomers, (D)-isomers,(L)-isomers, the racemic mixtures thereof, and other mixtures thereof,as falling within the scope of the invention. Additional asymmetriccarbon atoms may be present in a substituent such as an alkyl group. Allsuch isomers, as well as mixtures thereof, are intended to be includedin this invention.

If, for instance, a particular enantiomer of a compound made by methodsof the present invention is desired, it may be prepared by using anenantiomerically enriched reactant comprising an amino acid residue(e.g., a naturally occurring amino acid residue). Alternatively, theenantiomerically enriched reactants can be prepared by asymmetricsynthesis, or by derivation with a chiral auxiliary, where the resultingdiastereomeric mixture is separated and the auxiliary group cleaved toprovide the pure desired enantiomers. Alternatively, where the moleculecontains a basic functional group, such as amino, or an acidicfunctional group, such as carboxyl, diastereomeric salts are formed withan appropriate optically-active acid or base, followed by resolution ofthe diastereomers thus formed by fractional crystallization orchromatographic means well known in the art, and subsequent recovery ofthe pure enantiomers.

It will be understood that “substitution” or “substituted with” includesthe implicit proviso that such substitution is in accordance withpermitted valence of the substituted atom and the substituent, and thatthe substitution results in a stable compound, e.g., which does notspontaneously undergo transformation such as by rearrangement,fragmentation, decomposition, cyclization, elimination, or otherreaction.

The term “substituted” is also contemplated to include all permissiblesubstituents of organic compounds. In a broad aspect, the permissiblesubstituents include acyclic and cyclic, branched and unbranched,carbocyclic and heterocyclic, aromatic and nonaromatic substituents oforganic compounds. Illustrative substituents include, for example, thosedescribed herein above. The permissible substituents may be one or moreand the same or different for appropriate organic compounds. Forpurposes of this invention, the heteroatoms such as nitrogen may havehydrogen substituents and/or any permissible substituents of organiccompounds described herein which satisfy the valences of theheteroatoms. This invention is not intended to be limited in any mannerby the permissible substituents of organic compounds.

The phrase “protecting group”, as used herein, means temporarysubstituents which protect a potentially reactive functional group fromundesired chemical transformations. Examples of such protecting groupsinclude esters of carboxylic acids, silyl ethers of alcohols, andacetals and ketals of aldehydes and ketones, respectively. The field ofprotecting group chemistry has been reviewed (Greene, T. W.; Wuts, P. G.M. Protective Groups in Organic Synthesis, 2^(nd) ed.; Wiley: New York,1991). Protected forms of the inventive compounds are included withinthe scope of this invention. An “amino protecting group” means aprotecting group that protects an —NH—, —NH₂, or —N(R)H moiety, whereinR can be any hydrocarbyl or other substituent. Exemplary aminoprotecting groups include those that form carbamates such as9-fluorenylmethyloxycarbonyl (Fmoc), tertbutyloxycarbonyl (Boc),methyloxycarbonyl, and benzyloxycarbonyl (Cbz); amides such as benzoyl(Bz); and N-sulfonyl derivatives such as N-toluenesulfonamide (“tosyl”;Ts) or methanesulfonamide (Ms).

Two protecting groups may be referred to as “orthogonal” when a set ofreaction conditions sufficient to deprotect one protecting group isinsufficient to deprotect a second protecting group. In other words, insome reaction conditions, one protecting group is cleaved while anorthogonal protecting group (or an orthogonally protected moiety)remains intact. Exemplary pairs of orthogonal protecting groups include,but are not limited to, benzyl (Bn) and tert-butyloxycarbonyl (Boc);benzyl (Bn) and fluorenylmethylenoxy group (Fmoc); and tert-butyl andFmoc.

For purposes of the invention, the chemical elements are identified inaccordance with the Periodic Table of the Elements, CAS version,Handbook of Chemistry and Physics, 67th Ed., 1986-87, inside cover.

Other chemistry terms herein are used according to conventional usage inthe art, as exemplified by The McGraw-Hill Dictionary of Chemical Terms(ed. Parker, S., 1985), McGraw-Hill, San Francisco, incorporated hereinby reference). Unless otherwise defined, all technical and scientificterms used herein have the same meaning as commonly understood by one ofordinary skill in the art to which this invention pertains.

Salts of the compounds made by the methods of the invention includesalts derived from inorganic or organic acids including, for example,hydrochloric, hydrobromic, sulfuric, nitric, perchloric, phosphoric,formic, acetic, lactic, maleic, fumaric, succinic, tartaric, glycolic,salicylic, citric, methanesulfonic, benzenesulfonic, benzoic, malonic,trifluoroacetic, trichloroacetic, naphthalene-2-sulfonic, and otheracids.

As used herein, a protic solvent is a solvent that has a hydrogen atombound to an oxygen (as in a hydroxyl group) or a nitrogen (as in anamine group). In general terms, any solvent that contains labile H⁺ iscalled a protic solvent. The molecules of such solvents readily donateprotons (H⁺) to reagents. In contrast, an aprotic solvent is a solventthat does not have a hydrogen atom bound to an oxygen (as in a hydroxylgroup) or a nitrogen (as in an amine group), and it cannot donatehydrogen.

As used herein, a polar protic solvent is a protic solvent that willdissolve many salts. In general, these solvents have high dielectricconstants and high polarity. Non-limiting examples of polar proticsolvents include acetic acid, ammonia, ethanol, formic acid,isopropanol, methanol, n-butanol, nitromethane, n-propanol, t-butanol,and water.

As used herein, a polar aprotic solvent is a solvent that will dissolvemany salts, but lacks an acidic hydrogen; these solvents generally haveintermediate to high dielectric constants and polarity. Non-limitingexamples of polar aprotic solvents include acetone, acetonitrile,dichloromethane (DCM), dimethyl sulfoxide (DMSO), ethyl acetate,hexamethylphosphoric triamide (HMPT), N,N-dimethylformamide (DMF), andtetrahydrofuran (THF).

As used herein, a nonpolar aprotic solvent is a solvent that willdissolve many salts, but lacks an acidic hydrogen; these solventsgenerally have low dielectric constants and polarity. Non-limitingexamples of nonpolar aprotic solvents include benzene, chloroform,cyclohexane, diethyl ether, hexane, pentane, and toluene.

The nomenclature used to define the peptides is that typically used inthe art wherein the amino group at the N-terminus appears to the leftand the carboxyl group at the C-terminus appears to the right.

As used herein, the term “amino acid” includes both a naturallyoccurring amino acid and a non-natural amino acid. The term “aminoacid,” unless otherwise indicated, includes both isolated amino acidmolecules (i.e. molecules that include both, an amino-attached hydrogenand a carbonyl carbon-attached hydroxyl) and residues of amino acids(i.e. molecules in which either one or both an amino-attached hydrogenor a carbonyl carbon-attached hydroxyl are removed). The amino group canbe alpha-amino group, beta-amino group, etc. For example, the term“amino acid alanine” can refer either to an isolated alanine H-Ala-OH orto any one of the alanine residues H-Ala-, -Ala-OH, or -Ala-. Unlessotherwise indicated, all amino acids found in the compounds describedherein can be either in D or L configuration. An amino acid that is in Dconfiguration may be written such that “D” precedes the amino acidabbreviation. For example, “D-Arg” represents arginine in the Dconfiguration. The term “amino acid” includes salts thereof, includingpharmaceutically acceptable salts. Any amino acid can be protected orunprotected.

Protecting groups can be attached to an amino group (for examplealpha-amino group), the backbone carboxyl group, or any functionality ofthe side chain. As an example, phenylalanine protected by abenzyloxycarbonyl group (Z) on the alpha-amino group would berepresented as Z-Phe-OH.

With the exception of the N-terminal amino acid, all abbreviations ofamino acids (for example, Phe) in this disclosure stand for thestructure of —NH—C(R)(R′)—CO—, wherein R and R′ each is, independently,hydrogen or the side chain of an amino acid (e.g., R=benzyl and R′=H forPhe). Accordingly, phenylalanine is H-Phe-OH. The designation “OH” forthese amino acids, or for peptides (e.g., Lys-Val-Leu-OH) indicates thatthe C-terminus is the free acid. The designation “NH₂” in, for example,Lys-Val-Leu-NH₂ indicates that the C-terminus of the protected peptidefragment is amidated. Further, certain R and R′, separately, or incombination as a ring structure, can include functional groups thatrequire protection during the liquid phase synthesis.

Where the amino acid has isomeric forms, it is the L form of the aminoacid that is represented unless otherwise explicitly indicated as Dform, for example, D-Arg.

In certain embodiments, the amino protecting groups on the amino acidresidues include 9-fluorenylmethyloxycarbonyl group (Fmoc) andt-butyloxycarbonyl (Boc). The Fmoc group is removed from the aminoterminus with base while the Boc group is removed with acid. Inalternative embodiments, the amino protecting group may be formyl,acrylyl (Acr), benzoyl (Bz), acetyl (Ac), trifluoroacetyl, substitutedor unsubstituted groups of aralkyloxycarbonyl type, such as thebenzyloxycarbonyl (Z), p-chlorobenzyloxycarbonyl,p-bromobenzyloxycarbonyl, p-nitrobenzyloxycarbonyl,p-methoxybenzyloxycarbonyl, benzhydryloxycarbonyl,2(p-biphenylyl)isopropyloxycarbonyl,2-(3,5-dimethoxyphenyl)isopropyloxycarbonyl,p-phenylazobenzyloxycarbonyl, triphenylphosphonoethyloxycarbonyl or9-fluorenylmethyloxycarbonyl group (Fmoc), substituted or unsubstitutedgroups of alkyloxycarbonyl type, such as the tert-butyloxycarbonyl(Boc), tert-amyloxycarbonyl, diisopropylmethyloxycarbonyl,isopropyloxycarbonyl, ethyloxycarbonyl, allyloxycarbonyl, 2 methylsulphonylethyloxycarbonyl or 2,2,2-trichloroethyloxycarbonyl group,groups of cycloalkyloxycarbonyl type, such as thecyclopentyloxycarbonyl, cyclohexyloxycarbonyl, adamantyloxycarbonyl orisobornyloxycarbonyl group, and groups containing a hetero atom, such asthe benzenesulphonyl, p-toluenesulphonyl, mesitylenesulphonyl,methoxytrimethylphenylsulphonyl, 2-nitrobenzenesulfonyl,2-nitrobenzenesulfenyl, 4-nitrobenzenesulfonyl or 4-nitrobenzenesulfenylgroup.

Many amino acids bear reactive functional groups in the side chain. Incertain embodiments, such functional groups are protected in order toprevent the functional groups from reacting with the incoming aminoacid. The protecting groups used with these functional groups must bestable to the conditions of peptide synthesis, but may be removedbefore, after, or concomitantly with peptide bond formation.

As used herein, the term “peptide” refers to two or more amino acidscovalently linked by at least one amide bond (i.e. a bond between anamino group of one amino acid and a carboxyl group of another amino acidselected from the amino acids of the peptide fragment). The term“peptide” includes salts thereof, including pharmaceutically acceptablesalts.

Peptide Synthesis

The present invention provides methods of synthesizing Elamipretideusing NCA-based reagents. Elamipretide may be synthesized by convergentpeptide synthesis; e.g., a 2+2 peptide synthesis represented generallyby Scheme 1. Protecting groups PG¹-PG⁴ are defined herein.

Accordingly, in certain embodiments, the invention provides a method ofpreparing compound 1, or a salt thereof, comprising:

combining compound 1a and compound 1b under conditions sufficient toproduce compound 1, wherein:

compound 1 is represented by:

compound 1a is represented by:

compound 1b is represented by:

PG¹ represents alkyl, aryl, aralkyl, heteroaryl, heteroaralkyl,cycloalkyl, (cycloalkyl)alkyl, heterocycloalkyl, or(heterocycloalkyl)alkyl;

PG² represents H or an amino protecting group; and

R¹ represents H, alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl,—C(O)O— heteroaryl, —C(O)O-heteroaralkyl, or aralkyl.

In certain embodiments, PG¹ represents aralkyl or alkyl. For example,PG¹ may represent —CH₂Ph.

In certain embodiments, PG² represents an amino protecting group.Alternatively, PG² may represent H.

In certain preferred embodiments, R¹ represents H.

In certain embodiments, the method further comprises combining compound1 with an amino protecting reagent represented by PG³-X, thereby formingcompound 1c, wherein:

compound 1c is represented by:

PG³ represents an amino protecting group; and

X represents a leaving group.

In certain embodiments, PG³ is fluorenylmethyloxycarbonyl (Fmoc),tert-butyloxycarbonyl (Boc), benzoyl (Bz), tosyl (Ts), or, if PG² is anamino protecting group, a protecting group that can be removed underconditions that will not remove PG² (i.e., PG³ and PG² are “orthogonal”protecting groups); and X is chloride.

In certain embodiments, the method further comprises combining compound1c and a deprotection reagent under conditions sufficient to formcompound 1d, wherein:

compound 1d is represented by:

In certain such embodiments, PG¹ represents —CH₂Ph and the deprotectionreagent is H₂ and Pd/C.

In certain embodiments, the method further comprises combining compound1d and compound 2 under conditions sufficient to produce compound 10,wherein:

compound 10 is represented by:

compound 2 is represented by:

andPG⁴ represents an amino protecting group.

In certain embodiments, the conditions sufficient to produce compound 10comprise a peptide coupling reagent. Peptide coupling reagents that maybe used in the methods described herein are typically carbodiimidereagents. Examples of carbodiimide reagents include, but are not limitedto, N,N′-dicyclohexylcarbodiimide (DCC),1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC),N-cyclohexyl-N′-isopropylcarbodiimide (CIC),N,N′-diisopropylcarbodiimide (DIC), N-tert-butyl-N′-methylcarbodiimide(BMC), N-tert-butyl-N′-ethylcarbodiimide (BEC),bis[[4-(2,2-dimethyl-1,3-dioxolyl)]-methyl]carbodiimide (BDDC), andN,N-dicyclopentylcarbodiimide. In certain preferred embodiments, thepeptide coupling reagent is propane phosphonic acid anhydride,N,N′-di(isopropyl)carbodiimide, N,N′-di(cyclohexyl)carbodiimide,1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, or ethyl2-cyano-2-(hydroxyimino)acetate.

In certain embodiments, PG⁴ represents —C(O)O-alkyl, —C(O)O-aryl,—C(O)O— aralkyl, —C(O)O-heteroaryl, —C(O)O-heteroaralkyl, or aralkyl. Incertain preferred embodiments, PG⁴ represents —C(O)O-aralkyl.

In certain embodiments, the method further comprises making the NCAcompound 1b by combining compound 1e and a phosgene equivalent reagentunder conditions sufficient to produce compound 1b, wherein:

compound 1e is represented by:

In certain embodiments, the phosgene equivalent reagent is selected fromthe group consisting of phosgene, diphosgene, triphosgene, carbonyldiimidazole, and disuccinimidyl carbonate.

In certain embodiments, the method further comprises combining compound2a and compound 2b under conditions sufficient to produce compound 2,wherein:

compound 2a is represented by:

compound 2b is represented by:

and

R² represents H, alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl,—C(O)O— heteroaryl, —C(O)O-heteroaralkyl, or aralkyl.

In certain embodiments, R² is H.

In certain embodiments, the method further comprises combining compound2c and a phosgene equivalent reagent under conditions sufficient toproduce compound 2b, wherein:

compound 2c is represented by:

In certain embodiments, the phosgene equivalent reagent is selected fromthe group consisting of phosgene, diphosgene, triphosgene, carbonyldiimidazole, and disuccinimidyl carbonate.

In some embodiments, compound 1 has the following structure:

In some embodiments, compound 2 has the following structure:

In some embodiments, compound 10 has the following structure:

In other embodiments, the present invention provides a method ofpreparing compound 2, or a salt thereof, comprising:

combining compound 2a and compound 2b under conditions sufficient toproduce compound 2, wherein:

compound 2 is represented by:

compound 2a is represented by:

compound 2b is represented by:

PG⁴ represents an amino protecting group; and

R² represents H, alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl,—C(O)O— heteroaryl, —C(O)O-heteroaralkyl, or aralkyl.

In certain embodiments, R² represents H.

In certain embodiments, PG⁴ represents benzyloxycarbonyl (Cbz).

In certain embodiments, the method further comprises combining compound2c and a phosgene equivalent reagent under conditions sufficient toproduce compound 2b, wherein:

compound 2c is represented by:

In certain embodiments, the phosgene equivalent reagent is selected fromthe group consisting of phosgene, diphosgene, triphosgene, carbonyldiimidazole, and disuccinimidyl carbonate.

In some embodiments, the method further comprises comprising combiningcompound 2 and compound 1d under conditions sufficient to producecompound 10, wherein:

compound 10 is represented by:

compound 1d is represented by:

PG² represents H or an amino protecting group; and

PG³ represents an amino protecting group.

In some embodiments, PG² represents an amino protecting group.Alternatively, PG² may represent H.

In certain embodiments, PG³ is fluorenylmethyloxycarbonyl (Fmoc),tert-butyloxycarbonyl (Boc), benzoyl (Bz), tosyl (Ts)), or, if PG² is anamino protecting group, a protecting group that can be removed underconditions that will not remove PG² (i.e., PG³ and PG² are “orthogonal”protecting groups).

In certain embodiments, the conditions sufficient to produce compound 10comprise a peptide coupling reagent. Peptide coupling reagents that maybe used in the methods described herein are typically carbodiimidereagents. Examples of carbodiimide reagents include, but are not limitedto, N,N′-dicyclohexylcarbodiimide (DCC),1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDC),N-cyclohexyl-N′-isopropylcarbodiimide (CIC),N,N′-diisopropylcarbodiimide (DIC), N-tert-butyl-N′-methylcarbodiimide(BMC), N-tert-butyl-N′-ethylcarbodiimide (BEC),bis[[4-(2,2-dimethyl-1,3-dioxolyl)]-methyl]carbodiimide (BDDC), andN,N-dicyclopentylcarbodiimide. In certain preferred embodiments, thepeptide coupling reagent is propane phosphonic acid anhydride,N,N′-di(isopropyl)carbodiimide, N,N′-di(cyclohexyl)carbodiimide,1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, or ethyl2-cyano-2-(hydroxyimino)acetate.

In certain embodiments, compound 2 has the following structure:

In certain embodiments, compound 1d has the following structure:

In certain embodiments, compound 10 has the following structure:

Elamipretide may also be synthesized via a C-to-N linear convergentpeptide synthesis, e.g., represented generally by Scheme 2. In such aC-to-N linear peptide synthesis, an NCA reagent is used for each aminoacid installation. Protecting groups PG¹-PG⁴ are defined herein.

Accordingly, in other embodiments, the present invention provides amethod of preparing compound 2, or a salt thereof, comprising:

combining compound 2a and compound 2b under conditions sufficient toproduce compound 2, wherein:

compound 2 is represented by:

compound 2a is represented by:

compound 2b is represented by:

PG⁴ represents an amino protecting group; and

R² represents H, alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl,—C(O)O— heteroaryl, —C(O)O-heteroaralkyl, or aralkyl.

In certain embodiments, R² represents H.

In certain embodiments, PG⁴ represents benzyloxycarbonyl (Cbz).

In certain embodiments, the method further comprises combining compound2c and a phosgene equivalent reagent under conditions sufficient toproduce compound 2b, wherein:

compound 2c is represented by:

In certain embodiments, the phosgene equivalent reagent is selected fromthe group consisting of phosgene, diphosgene, triphosgene, carbonyldiimidazole, and disuccinimidyl carbonate.

In certain embodiments, the method further comprises combining compound2 and compound 3a under conditions sufficient to produce compound 3,wherein:

compound 3 is represented by:

compound 3a is represented by:

and

-   R³ represents H, alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl,    —C(O)O-heteroaryl, —C(O)O-heteroaralkyl, or aralkyl.

In certain embodiments, R³ represents H.

In certain embodiments, the method further comprises combining compound3b and a phosgene equivalent reagent under conditions sufficient toproduce compound 3a, wherein:

compound 3b is represented by:

In certain embodiments, the phosgene equivalent reagent is selected fromthe group consisting of phosgene, diphosgene, triphosgene, carbonyldiimidazole, and disuccinimidyl carbonate.

In certain embodiments, the method further comprises combining compound3 and compound 4a under conditions sufficient to produce compound 4,wherein:

compound 4 is represented by:

compound 4a is represented by:

R⁴ represents H, alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl,—C(O)O— heteroaryl, —C(O)O-heteroaralkyl, or aralkyl; and

PG² represents H or an amino protecting group.

In certain embodiments, R⁴ represents H.

In certain embodiments, PG² represents H.

In certain embodiments, the method further comprises combining compound4b and a phosgene equivalent reagent under conditions sufficient toproduce compound 4a, wherein:

compound 4b is represented by:

In certain embodiments, the phosgene equivalent reagent is selected fromthe group consisting of phosgene, diphosgene, triphosgene, carbonyldiimidazole, and disuccinimidyl carbonate.

In certain embodiments, compound 3 has the following structure:

In certain embodiments, compound 4 has the following structure:

Elamipretide may also be synthesized via alternative linear convergentpeptide synthesis routes, such as the route represented generally byScheme 3. Protecting groups PG¹-PG⁵ are defined herein.

Accordingly, in certain embodiments, the invention provides a method ofpreparing compound 5, or a salt thereof, comprising:

combining compound 5a and compound 3a under conditions sufficient toproduce compound 5, wherein:

compound 5 is represented by:

compound 5a is represented by:

compound 3a is represented by:

PG⁴ represents an amino protecting group;

PG⁵ represents alkyl, aryl, aralkyl, heteroaryl, heteroaralkyl,cycloalkyl, (cycloalkyl)alkyl, heterocycloalkyl, or(heterocycloalkyl)alkyl; and

R³ represents H, alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl,—C(O)O— heteroaryl, —C(O)O-heteroaralkyl, or aralkyl.

In certain embodiments, PG⁵ represents aralkyl or alkyl. Preferably, PG⁵represents —CH₂Ph.

In certain embodiments, PG⁴ represents benzyloxycarbonyl (Cbz).

In certain embodiments, R³ represents H.

In certain embodiments, the method further comprises combining compound3b and a phosgene equivalent reagent under conditions sufficient toproduce compound 3a, wherein:

compound 3b is represented by:

In certain embodiments, the phosgene equivalent reagent is selected fromthe group consisting of phosgene, diphosgene, triphosgene, carbonyldiimidazole, and disuccinimidyl carbonate.

In certain embodiments, the method further comprises combining compound5 and compound 1b under conditions sufficient to produce compound 6,wherein:

compound 6 is represented by:

compound 1b is represented by:

PG² represents H or an amino protecting group; and

R¹ represents H, alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl,—C(O)O— heteroaryl, —C(O)O-heteroaralkyl, or aralkyl.

In some embodiments, PG² represents an amino protecting group.Alternatively, PG² may represent H.

In some embodiments, R¹ represents H.

In certain embodiments, the method further combining compound 1e and aphosgene equivalent reagent under conditions sufficient to producecompound 1b, wherein:

compound 1e is represented by:

In certain embodiments, the phosgene equivalent reagent is selected fromthe group consisting of phosgene, diphosgene, triphosgene, carbonyldiimidazole, and disuccinimidyl carbonate.

In certain embodiments, compound 5 has the following structure:

In certain embodiments compound 6 has the following structure:

Elamipretide and other peptide intermediates produced by the methods ofthe invention can be collected or purified by a routine method, forexample, chromatography, such as gel filtration chromatography, ionexchange column chromatography, affinity chromatography, reverse phasecolumn chromatography, and HPLC, ammonium sulfate fractionation,ultrafiltration, and immunoadsorption.

EQUIVALENTS

Having now fully described the present invention in some detail by wayof illustration and example for purposes of clarity of understanding, itwill be obvious to one of ordinary skill in the art that the same can beperformed by modifying or changing the invention within a wide andequivalent range of conditions, formulations and other parameterswithout affecting the scope of the invention or any specific embodimentthereof, and that such modifications or changes are intended to beencompassed within the scope of the appended claims.

INCORPORATION BY REFERENCE

All U.S. patents and U.S. and PCT published patent applicationsmentioned in the description above are incorporated by reference hereinin their entirety.

We claim:
 1. A method of preparing compound 1, or a salt thereof,comprising: combining compound 1a and compound 1b under conditionssufficient to produce compound 1, wherein: compound 1 is represented by:

compound 1a is represented by:

compound 1b is represented by:

PG¹ represents alkyl, aryl, aralkyl, heteroaryl, heteroaralkyl,cycloalkyl, (cycloalkyl)alkyl, heterocycloalkyl, or(heterocycloalkyl)alkyl; PG² represents H or an amino protecting group;and R¹ represents H, alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl,—C(O)O-heteroaryl, —C(O)O-heteroaralkyl, or aralkyl.
 2. The method ofclaim 1, wherein PG¹ represents aralkyl or alkyl.
 3. The method of claim2, wherein PG¹ represents —CH₂Ph.
 4. The method of any one of claims1-3, wherein PG² represents an amino protecting group.
 5. The method ofany one of claims 1-3, wherein PG² represents H.
 6. The method of anyone of claims 1-5, wherein R¹ represents H.
 7. The method of any one ofclaims 1-6, further comprising combining compound 1 with an aminoprotecting reagent represented by PG³-X, thereby forming compound 1c,wherein: compound 1c is represented by:

PG³ represents an amino protecting group; and X represents a leavinggroup.
 8. The method of claim 7, wherein PG³ isfluorenylmethyloxycarbonyl (Fmoc), tert-butyloxycarbonyl (Boc), benzoyl(Bz), tosyl (Ts), or, if PG² is an amino protecting group, a protectinggroup that can be removed under conditions that will not remove PG²; andX is chloride.
 9. The method of claim 7 or 8, further comprisingcombining compound 1c and a deprotection reagent under conditionssufficient to form compound 1d, wherein: compound 1d is represented by:


10. The method of claim 9, wherein PG¹ represents —CH₂Ph and thedeprotection reagent is H₂ and Pd/C.
 11. The method of claim 9 or 10,further comprising combining compound 1d and compound 2 under conditionssufficient to produce compound 10, wherein: compound 10 is representedby:

compound 2 is represented by:

and PG⁴ represents an amino protecting group.
 12. The method of claim11, wherein the conditions sufficient to produce compound 10 comprise apeptide coupling reagent.
 13. The method of claim 12, wherein thepeptide coupling reagent is propane phosphonic acid anhydride,N,N′-di(isopropyl)carbodiimide, N,N′-di(cyclohexyl)carbodiimide,1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, or ethyl2-cyano-2-(hydroxyimino)acetate.
 14. The method of any one of claims11-13, wherein PG⁴ represents —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl,—C(O)O-heteroaryl, —C(O)O-heteroaralkyl, or aralkyl.
 15. The method ofclaim 14, wherein PG⁴ represents —C(O)O-aralkyl.
 16. The method of anyone of claims 1-15, further comprising combining compound 1e and aphosgene equivalent reagent under conditions sufficient to producecompound 1b, wherein: compound 1e is represented by:


17. The method of claim 16, wherein the phosgene equivalent reagent isselected from the group consisting of phosgene, diphosgene, triphosgene,carbonyl diimidazole, and disuccinimidyl carbonate.
 18. The method ofany one of claims 11-17, further comprising combining compound 2a andcompound 2b under conditions sufficient to produce compound 2, wherein:compound 2a is represented by:

compound 2b is represented by:

and R² represents H, alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl,—C(O)O-heteroaryl, —C(O)O-heteroaralkyl, or aralkyl.
 19. The method ofclaim 18, wherein R² is H.
 20. The method of 18 or 19, furthercomprising combining compound 2c and a phosgene equivalent reagent underconditions sufficient to produce compound 2b, wherein: compound 2c isrepresented by:


21. The method of claim 20, wherein the phosgene equivalent reagent isselected from the group consisting of phosgene, diphosgene, triphosgene,carbonyl diimidazole, and disuccinimidyl carbonate.
 22. The method ofany one of claims 1-21, wherein compound 1 has the following structure:


23. The method of any one of claims 11-22, wherein compound 2 has thefollowing structure:


24. The method of any one of claims 11-23, wherein compound 10 has thefollowing structure:


25. A method of preparing compound 2, or a salt thereof, comprising:combining compound 2a and compound 2b under conditions sufficient toproduce compound 2, wherein: compound 2 is represented by:

compound 2a is represented by:

compound 2b is represented by:

PG⁴ represents an amino protecting group; and R² represents H, alkyl,—C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl, —C(O)O-heteroaryl,—C(O)O-heteroaralkyl, or aralkyl.
 26. The method of claim 25, wherein R²represents H.
 27. The method of claim 25 or 26, wherein PG⁴ representsbenzyloxycarbonyl (Cbz).
 28. The method of any one of claims 25-27,further comprising combining compound 2c and a phosgene equivalentreagent under conditions sufficient to produce compound 2b, wherein:compound 2c is represented by:


29. The method of claim 28, wherein the phosgene equivalent reagent isselected from the group consisting of phosgene, diphosgene, triphosgene,carbonyl diimidazole, and disuccinimidyl carbonate.
 30. The method ofany one of claims 25-29, further comprising combining compound 2 andcompound 1d under conditions sufficient to produce compound 10, wherein:compound 10 is represented by:

compound 1d is represented by:

PG² represents H or an amino protecting group; and PG³ represents anamino protecting group.
 31. The method of claim 30, wherein PG²represents an amino protecting group.
 32. The method of claim 30,wherein PG² represents H.
 33. The method of any one of claims 30-32,wherein PG³ is fluorenylmethyloxycarbonyl (Fmoc), tert-butyloxycarbonyl(Boc), benzoyl (Bz), tosyl (Ts), or, if PG² is an amino protectinggroup, a protecting group that can be removed under conditions that willnot remove PG².
 34. The method of any one of claims 30-33, wherein theconditions sufficient to produce compound 10 comprise a peptide couplingreagent.
 35. The method of claim 34, wherein the peptide couplingreagent is propane phosphonic acid anhydride,N,N′-di(isopropyl)carbodiimide, N,N′-di(cyclohexyl)carbodiimide,1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, or ethyl2-cyano-2-(hydroxyimino)acetate.
 36. The method of any one of claims25-35, wherein compound 2 has the following structure:


37. The method of any one of claims 30-36, wherein compound 1d has thefollowing structure:


38. The method of any one of claims 30-37, wherein compound 10 has thefollowing structure:


39. The method of any one of claims 25-29, further comprising combiningcompound 2 and compound 3a under conditions sufficient to producecompound 3, wherein: compound 3 is represented by:

compound 3a is represented by:

and R³ represents H, alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl,—C(O)O-heteroaryl, —C(O)O-heteroaralkyl, or aralkyl.
 40. The method ofclaim 39, wherein R³ represents H.
 41. The method of claim 39 or 40,further comprising combining compound 3b and a phosgene equivalentreagent under conditions sufficient to produce compound 3a, wherein:compound 3b is represented by:


42. The method of claim 41, wherein the phosgene equivalent reagent isselected from the group consisting of phosgene, diphosgene, triphosgene,carbonyl diimidazole, and disuccinimidyl carbonate.
 43. The method ofany one of claims 39-42, further comprising combining compound 3 andcompound 4a under conditions sufficient to produce compound 4, wherein:compound 4 is represented by:

compound 4a is represented by:

R⁴ represents H, alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl,—C(O)O-heteroaryl, —C(O)O-heteroaralkyl, or aralkyl; and PG² representsH or an amino protecting group.
 44. The method of claim 43, wherein R⁴represents H.
 45. The method of claim 43, wherein PG² represents H. 46.The method of any one of claims 43-45, further comprising combiningcompound 4b and a phosgene equivalent reagent under conditionssufficient to produce compound 4a, wherein: compound 4b is representedby:


47. The method of claim 46, wherein the phosgene equivalent reagent isselected from the group consisting of phosgene, diphosgene, triphosgene,carbonyl diimidazole, and disuccinimidyl carbonate.
 48. The method ofany one of claims 39-47, wherein compound 3 has the following structure:


49. The method of any one of claims 39-48, wherein compound 4 has thefollowing structure:


50. A method of preparing compound 5, or a salt thereof, comprising:combining compound 5a and compound 3a under conditions sufficient toproduce compound 5, wherein: compound 5 is represented by:

compound 5a is represented by:

compound 3a is represented by:

PG⁴ represents an amino protecting group; PG⁵ represents alkyl, aryl,aralkyl, heteroaryl, heteroaralkyl, cycloalkyl, (cycloalkyl)alkyl,heterocycloalkyl, or (heterocycloalkyl)alkyl; and R³ represents H,alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl, —C(O)O-heteroaryl,—C(O)O-heteroaralkyl, or aralkyl.
 51. The method of claim 50, whereinPG⁵ represents aralkyl or alkyl.
 52. The method of claim 51, wherein PG⁵represents —CH₂Ph.
 53. The method of any one of claims 50-52, whereinPG⁴ represents benzyloxycarbonyl (Cbz).
 54. The method of any one ofclaims 50-53, wherein R³ represents H.
 55. The method of any one ofclaims 50-54, further comprising combining compound 3b and a phosgeneequivalent reagent under conditions sufficient to produce compound 3a,wherein: compound 3b is represented by:


56. The method of claim 55, wherein the phosgene equivalent reagent isselected from the group consisting of phosgene, diphosgene, triphosgene,carbonyl diimidazole, and disuccinimidyl carbonate.
 57. The method ofany one of claims 50-56, further comprising combining compound 5 andcompound 1b under conditions sufficient to produce compound 6, wherein:compound 6 is represented by:

compound 1b is represented by:

PG² represents H or an amino protecting group; and R¹ represents H,alkyl, —C(O)O-alkyl, —C(O)O-aryl, —C(O)O-aralkyl, —C(O)O-heteroaryl,—C(O)O-heteroaralkyl, or aralkyl.
 58. The method of claim 57, whereinPG² represents an amino protecting group.
 59. The method of claim 57,wherein PG² represents H.
 60. The method of any one of claims 57-59,wherein R¹ represents H.
 61. The method of any one of claims 57-60,further comprising combining compound 1e and a phosgene equivalentreagent under conditions sufficient to produce compound 1b, wherein:compound 1e is represented by:


62. The method of claim 61, wherein the phosgene equivalent reagent isselected from the group consisting of phosgene, diphosgene, triphosgene,carbonyl diimidazole, and disuccinimidyl carbonate.
 63. The method ofany one of claims 50-62, wherein compound 5 has the following structure:


64. The method of any one of claims 50-63, wherein compound 6 has thefollowing structure: